1. Field of the Invention
This invention relates to a process for the production of fibrous activated carbon having high adsorptivity and excellent mechanical properties from a starting material of acrylonitrile-based fiber.
2. Description of the Prior Art
Fibrous activated carbon in a tow, fabric or felt form has received increasing attention in recent years as an adsorbent, filter, or the like, for use, e.g., in solvent recovery equipment.
For the production of such fibrous activated carbon, a method in which a cellulose based fiber, phenol resin fiber, acrylonitrile-based fiber, or the like is carbonized and activated has heretofore been proposed. Of these fibrous activated carbons, the fibrous activated carbon produced from acrylonitrile-based fiber has unique adsorption capabilities because of the nitrogen atoms contained therein and, furthermore, has excellent mechanical strength. Thus, it has various applications.
A process for the production of fibrous activated carbon from a starting material of acrylonitrile-based fiber usually comprises the following steps:
A preoxidation step wherein the feed fiber is preoxidized in an oxidizing gas atmosphere, such as oxygen or the like, at a temperature of from 150.degree. C. to 300.degree. C.; and an activation step wherein the preoxidized fiber obtained in the preoxidation step is treated in an atmosphere of steam, carbon dioxide gas or the like at a temperature of from 700.degree. C. to 1,000.degree. C.
Of these steps, the preoxidation step exerts a significant influence on the yield, adsorptivity, and mechanical properties of the final product.
In the preoxidation step, an abrupt generation of heat of reaction is involved, since the heat of reaction is evolved as the cyclization and crosslinking of molecules constituting the acrylonitrile based fiber proceeds. As a result, when fibers having a diameter of more than 1.5 denier the surface of the fibers melts and softens, and fibers adjacent to each other easily stick together (this phenomenon is hereinafter referred to as "coalescence"). In order to avoid such coalescence, the preoxidation has heretofore been carried out at low temperatures over very long periods of time.
The occurrence of such coalescence in the preoxidation step gives rise to the problems that fibers are easily cut, and therefore stable operation becomes difficult. Furthermore, such coalescence exerts undesirable influences on the yield of fibrous activated carbon and performance of the fibrous activated carbon obtained.
As the comonomer content of fiber is increased and the temperature at which the heat-treatment is carried out is raised, the coalescence of fibers at the oxidation step occurs more easily. Therefore, when an acrylonitrile-based fiber having a high comonomer content, that is, more than about 6 wt% is used as a starting material, it is necessary to carry out the preoxidation or heat-treatment at low temperatures for long periods of time. Thus, in the prior art process for the production of fibrous activated carbon, the time required for the oxidation step constitutes about 80% of the total time required for the total process. This is very inefficient and is a main factor for the increasing production costs of fibrous activated carbon.